1. Field of the Invention
The present invention relates to a halftone image recording method, especially a halftone image recording method using recording dots having a plurality of concentrations for the same color.
2. Related Art
A number of halftone image recording methods used in conventional printers, e.g., an inkjet printer, have been proposed. For example, an analogue modifying method, which varies sizes of ink dots by changing pulse height or pulse width of an electrical signal applied to an inkjet head, a digital modifying method, which varies the number of dots formed in a dot matrix without changing the size of the dots, and a digital and analogue modifying method which is a combination of the analogue and the digital methods and which varies the sizes of the dots and the number of the dots in the dot matrix, are well known. However, the smallest dot formed by the inkjet head is not so small that the density change looks big in the highlight region (low density region), especially in the boundary region between the region having no dots and the region having ink dots. Therefore, in the boundary region, the quality of the image may not be acceptable. In other words, even though the density in the original is constant, the image signal output from the imaging device is not constant because of variations in the input system, e.g., the imaging device, the irradiating conditions, etc. As a result, the ink dots are not recorded uniformly, and the recorded image may look rough. Making the size of the dot matrix larger is proposed to resolve the above-mentioned shortcoming. However, if a large dot matrix is used, the resolution of the image gets worse, and the pattern of the dot matrix is emphasized, and the image quality is not satisfactorily improved after all.
Thus, a halftone image recording method using recording dots having a plurality of concentrations for one color is provided for improving density characteristic in the highlight or boundary regions and for making the number of gradations larger.
However, if only two concentrations of ink for one color are provided, the dot matrix size must be large if it is desired to make the number of gradations larger, considering that ink dots are recorded on the same position over one another. Therefore, the resolution of the image is still not what is desired, and the pattern or the texture of the dot matrix still stands out.
If three or more concentrations of ink are provided for one color, many gradations could be obtained without making the matrix size larger. But there are some shortcomings to such an approach, e.g., increasing the kinds of ink and the number of inkjet heads.
On the other hand, in the case of color image formation, if a plurality of different concentrations of ink are provided for each color, many kinds of ink dots, e.g., cyan thick and thin ink, magenta thick and thin ink, yellow thick and thin ink, are recorded on the same position, and depending on the case, black thick and thin ink may also be recorded on the same position. In this case, the amount of ink recorded on the same position exceeds the capacity of ink accommodation of the recording paper, and there is a possibility that the excess ink may flow on the recording paper.
For the yellow inks, the brightness range of the yellow ink, from the brightness level of white to that of maximum visual saturation of yellow, is narrower than that of cyan or magenta. Therefore, even if the number of concentrations of ink for yellow is increased, the resulting improvement of the image quality is not all that is desired.
In other words, if inks of many concentrations are provided, the required controlling circuit becomes complicated because of the increase in number of the inkjet heads, and the image quality may be degraded as a result of flowing of ink.